Aluminum production cell and cathode

ABSTRACT

A cell for the production of aluminium by the electrolysis of an aluminium compound dissolved in a molten electrolyte, in which an outer mechanical structure forming an outer shell ( 21 ) houses therein one or more inner electrically-conductive cathode holder shells or plates ( 31 ) which contain a cathode mass ( 32 ) and is/are connected electrically to the busbar. The cathode mass ( 32 ) has an aluminium-wettable top surface ( 37 ), preferably at a slope forming a drained cathode. The inner cathode holder shell or shells ( 31 ) is/are separated from the outer shell ( 21 ) by an electric and thermic insulation ( 40 ), the cathode holder shell(s) ( 31 ) also serving to distribute current uniformly to the cathode mass ( 32 ). The or each cathode ( 30 ) formed by the cathode holder shell ( 31 ) and cathode mass ( 32 ) is removable from the cell as a unit.

FIELD OF THE INVENTION

The invention relates to the production of aluminium by the electrolysisof an aluminium compound dissolved in a molten electrolyte, for examplealumina dissolved in a molten fluoride-based electrolyte. It concerns inparticular, but not exclusively, cells of the type having a drainedcathode having sloping drained cathode surfaces. The invention alsorelates to cathodes of such cells, their manufacture, and methods ofoperating the cells to produce aluminium.

BACKGROUND OF THE INVENTION

The technology for the production of aluminium by the electrolysis ofalumina, dissolved in molten cryolite containing salts, at temperaturesaround 950° C. is more than one hundred years old.

This process, conceived almost simultaneously by Hall and Héroult, hasnot evolved as much as other electrochemical processes, despite thetremendous growth in the total production of aluminium that in fiftyyears has increased almost one hundred fold. The process and the celldesign have not undergone any great change or improvement andcarbonaceous materials are still used as electrodes and cell linings.

The electrolytic cell trough is typically made of a steel shell providedwith an insulating lining of refractory material covered by prebakedanthracite-graphite or all graphite carbon blocks at the cell floorbottom which acts as cathode and to which the negative pole of a directcurrent source is connected by means of steel conductor bars embedded inthe carbon blocks. The side walls are also covered with prebakedanthracite-graphite carbon plates or silicon carbide plates.

The anodes are still made of carbonaceous material and must be replacedevery few weeks. The operating temperature is still approximately 950°C. in order to have a sufficiently high rate of dissolution of aluminawhich decreases at lower temperatures and to have a higher conductivityof the electrolyte.

The carbonaceous materials used in Hall-Héroult cells as cell liningdeteriorate under the existing adverse operating conditions and limitthe cell life.

The anodes have a very short life because during electrolysis the oxygenwhich should evolve on the anode surface combines with the carbon toform CO₂ and small amounts of CO. The actual consumption of the anode isapproximately 450 kg/ton of aluminium produced which is more than ⅓higher than the theoretical amount.

The carbon lining of the cathode bottom has a useful life of a few yearsafter which the operation of the entire cell must be stopped and thecell relined at great cost. Despite an aluminium pool having a thicknessof 10 to 20 cm maintained over the cathode, the deterioration of thecathode carbon blocks cannot be avoided because of penetration of sodiuminto the carbon which by chemical reaction and intercalation causesswelling, deformation and disintegration of the cathode carbon blocks,and because of penetration of cryolite and liquid aluminium.

The carbonaceous blocks of the cell side wall do not resist oxidationand attack by cryolite and a layer of solidified cryolite has to bemaintained on the cell side walls to protect them. In addition, whencells are rebuilt, there are problems of disposal of the carbon cathodeswhich contain toxic compounds including cyanides.

Another major drawback, however, is due to the fact that irregularelectromagnetic forces create waves in the molten aluminium pool and theanode-cathode distance (ACD), also called interelectrode gap (IEG), mustbe kept at a safe minimum value of approximately 50 mm to avoid shortcircuiting between the aluminium cathode and the anode or reoxidation ofthe metal by contact with the CO₂ gas formed at the anode surface,leading to a lower current efficiency.

The high electrical resistivity of the electrolyte, which is about 0.4ohm. cm., causes a voltage drop which alone represents more than 40% ofthe total voltage drop with a resulting high energy consumption which isclose to 13 kWh/kgAl in the most modern cells. The cost of energyconsumption has become an even bigger item in the total manufacturingcost of aluminium since the oil crisis, and has decreased the rate ofgrowth of this important metal.

In the second largest electrochemical industry following aluminium,namely the caustic and chlorine industry, the invention of thedimensionally stable anodes (DSA®) based on noble metal activatedtitanium metal, which were developed around 1970, permitted arevolutionary progress in the chlorine cell technology resulting in asubstantial increase in cell energy efficiency, in cell life and inchlorine-caustic purity. The substitution of graphite anodes with DSA®increased drastically the life of the anodes and reduced substantiallythe cost of operating the cells. Rapid growth of the chlorine causticindustry was retarded only by ecological concerns.

In the case of aluminium production, pollution is not due to thealuminium produced, but to the materials and the manufacturing processesused and to the cell design and operation.

However, progress has been reported in the operation of modern aluminiumplants which utilize cells where the gases emanating from the cells arein large part collected and adequately scrubbed and where the emissionof highly polluting gases during the manufacture of the carbon anodesand cathodes is carefully controlled.

While progress has been reported in the use of carbon cathodes to whichhave been applied coatings or layers of new aluminium wettable materialswhich are also a barrier to sodium penetration during electrolysis, verylittle progress has been achieved in design of cathodes for aluminiumproduction cells with a view to improving the overall cell efficiency,simplifying assembly of the cathodes in the cell, simplifying theremoval and disposal of used cathodes, as well as restraining movementof the molten aluminium in order to reduce the interelectrode gap andthe rate of wear of its surface.

U.S. Pat. No. 3,202,600 (Ransley) proposed the use of refractory boridesand carbides as cathode materials, including a drained cathode celldesign wherein a wedge-shaped consumable carbon anode was suspendedfacing a cathode made of plates of refractory boride or carbide inV-configuration.

U.S. Pat. No. 3,400,061 (Lewis et al) and U.S. Pat. No. 4,602,990(Boxall et al) disclose aluminium electrowinning cells with slopeddrained cathodes arranged with the cathodes and facing anode surfacessloping across the cell. In these cells, the molten aluminium flows downthe sloping cathodes into a median longitudinal groove along the centerof the cell, or into lateral longitudinal grooves along the cell sides,for collecting the molten aluminium and delivering it to a sump.

U.S. Pat. No. 4,544,457 (Sane et al) proposed a drained cathodearrangement in which the surface of a carbon cathode block was coveredwith a sheath that maintained stagnant aluminium on its surface in orderto reduce wear. In this design, the cathode block stands on the cellbottom.

U.S. Pat. No. 5,203,971 (de Nora et al) discloses an aluminiumelectrowinning cell having a partly refractory and partly carbon basedcell lining. The carbon-based part of the cell bottom may be recessed inrespect to the refractory part, which assists in reducing movement ofthe aluminium pool.

U.S. Pat. No. 3,856,650 (Kugler) proposed lining a carbon cell bottomwith a ceramic coating upon which parallel rows of tiles are placed, inthe molten aluminium, in a grating-like arrangement in an attempt toreduce wear due to movements of the aluminium pool.

To restrict movement in a “deep” cathodic pool of molten aluminium, U.S.Pat. No. 4,824,531 (Duruz et al) proposed filling the cell bottom with apacked bed of loose pieces of refractory material. Such a design hasmany potential advantages but, because of the risk of forming a sludgeby detachment of particles from the packed bed, the design has not foundacceptance. U.S. Pat. No. 4,443,313 (Dewing et al) sought to avoid thisdisadvantage of the previously mentioned loose packed bed by providing amonolayer of closely packed small ceramic shapes such as balls, tubes orhoneycomb tiles.

An improvement described in U.S. Pat. No. 5,472,578 (de Nora) consistedin using grid-like bodies which could form a drained cathode surface andsimultaneously restrain movement in the aluminium pool.

U.S. Pat. No. 5,316,718 and WO 93/25731 (both in the name of Sekhar etal) proposed coating components with a slurry-applied coating ofrefractory boride, which proved excellent for cathode applications.These publications included a number of novel drained cathodeconfigurations, for example including designs where a cathode body withan inclined upper drained cathode surface is placed on or secured to thecell bottom.

In U.S. Pat. No. 5,362,366 (de Nora et al), a double-polar anode-cathodearrangement was disclosed wherein cathode bodies were suspended from theanodes permitting removal and reimmersion of the assembly duringoperation, such assembly also operating with a drained cathode.

U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cellhaving upwardly extending cathodes facing and surrounded by orin-between anodes having a relatively large inwardly-facing active anodesurface area. In some embodiments, electrolyte circulation was achievedusing a tubular anode with suitable openings.

WO 96/07773 (de Nora) proposed a new cathode design for a drainedcathode, where grooves or recesses were incorporated in the surface ofblocks forming the cathode surface in order to channel the drainedproduct aluminium.

As regards the supply of current to the cathodes, the most usualarrangement is to have horizontal cathode current supply bars whichextend across the cell bottom and protrude from its sides (see forexample U.S. Pat. No. 4,834,531 referred to above). These horizontalcurrent supply bars conveniently are located in grooves in the bottomsurfaces of the cathode blocks, as illustrated in WO 96/07773 (de Nora),and extend all the way across the cell bottom.

By these means, current is supplied to the cathodes from externalbuswork extending along the sides of the cells. After passing throughthe electrolysis cell by ionic conduction, the current is taken up bythe anodes suspended by an anode suspension and current-supplysuperstructure. Conventionally, this superstructure supplies current toa line of cells whose cathodes and anodes are all connected together tocathode and anode buswork.

Proposals have also been made to supply current to the cathodes viagenerally vertical current collector bars. These proposals—see forexample U.S. Pat. No. 5,071,533 (de Nora et al) and U.S. Pat. No.4,613,418 (Dewing et al)—have concerned non-carbon cell bottoms, whereit was intended to replace the conventional carbon cathode with anon-conductive refractory material such as various grades of compactedparticulate fused alumina. In this case, the current collector barserves to deliver current to a pool or layer of aluminium. Suchproposals however encountered various difficulties, so that carboncathodes remain as industry standard and are particularly advantageouswhen coated with a slurry-applied layer of an aluminium-wettable boride.

EP-A-0 345 959 (Nebell et al) discloses a potline for the electrolyticproduction of aluminium which comprises rows of reduction cells withcells arranged transversely in each row, each cell having at least oneconductor projecting through the bottom of the cell for each carboncathode block. About half of the electric current is conducted to acathode collector busbar and the other half to another collector busbarfrom where the current is carried to the next cell via two busbars.

U.S. Pat. No. 3,110,660 (Miller) discloses an electrolytic cell for theproduction of aluminium wherein the cathode comprises a plurality ofcarbon slabs which are located along the bottom of the cell on ametallic support pan for conducting away the current. The currentcollecting pan has lateral extensions extending through the sidewalls ofthe cell and welded to external steel conductor bars.

WO 97/48838 (Juric et al) whose priority date is Jun. 18, 1996 and whichwas published on Dec. 24, 1997, discloses an electrolytic reduction cellwhose cathode comprises a carbonaceous cathode block having a pluralityof electrical contact plugs mounted in electrical contact to and above acollector plate for collecting current from the cathode blocks. Thecollector plate is joined to or integrally formed with collector barsextending through the sidewalls.

While the foregoing references indicate continued efforts to improve theoperation of molten cell electrolysis operations, none suggest theinvention and there have been no acceptable proposals for improving theefficiency of the supply of electric current to a cathode body, whilesimplifying assembly and replacement of the cathodes, and at the sametime facilitating the implementation of a drained cathode configuration.

OBJECTS OF THE INVENTION

One object of the invention is to overcome problems inherent in theconventional design of cells used in the electrowinning of aluminium bythe electrolysis of an aluminium compound such as alumina dissolved inmolten electrolyte for example fluoride-based melts in particularcryolite, notably by improving the efficiency of the supply of electriccurrent to a cathode body.

Another object of the invention is to permit more efficient celloperation by modifying the design of the cathode to improve thedistribution of electric current to the cathode.

A further object of the invention is to provide a novel cathodepermitting improved distribution of electric current, which can beeasily produced and fitted in the cell, and which simplifies dismantlingof the cell to replace or refurbish the cathodes.

A yet further object of the invention is to provide an improved cathodewhich facilitates the implementation of a drained cell configuration.

Yet another object of the invention is to provide a system forinterconnecting aluminium production cells enabling reduction of thetotal floorspace needed for a given production, by providing asimplified buswork arrangement while maintaining ease of access to thecells for maintenance.

A yet further object of the invention is to provide a cathode of noveldesign enabling drained cathode operation where ease of removal of theanodically produced gases is combined with ease of collection of theproduct aluminium.

An even further object of the invention is to provide an aluminiumproduction cell in which fluctuating electric currents that produce avariable electromagnetic field are reduced or eliminated therebyreducing or eliminating the adverse effects that lead to a reduction ofthe cell efficiency.

SUMMARY OF THE INVENTION

One main aspect of the invention concerns a cell for the production ofaluminium by the electrolysis of an aluminium compound dissolved in amolten electrolyte, in which the electric current to the cathode arrivesthrough an inner cathode holder shell or plate (hereinafter sometimesreferred to simply as “inner shell”) placed between the cathode and theouter shell, usually made of steel.

In this cell, an inner cathode holder shell (or plate) of metal orsuitable electrically conductive material is placed between the cathodesurface and the outer shell, the inner shell serving to distributecurrent uniformly to the cathode and being connected directly to thenegative busbar.

More precisely, the invention concerns a cell for the production ofaluminium by the electrolysis of an aluminium compound dissolved in amolten electrolyte, in which an electrically-conductive inner cathodeholder shell or plate, electrically connected to the negative busbar, islocated inside the outer shell of the cell, the inner shell containingand/or supporting a cathode mass and being separated from the outershell by an electric and thermic insulating mass, the inner shell alsoserving to distribute current to the cathode mass.

In other terms, in the aluminium production cell according to theinvention, an outer mechanical structure forming an outer shell housestherein an inner electrically-conductive shell (or plate) which containsand/or supports a cathode mass. This plate or shell is connectedelectrically to the busbar and the inner cathode holder plate or shellis separated from the outer shell by an electric and thermic insulation.This inner plate or shell thus serves to support the cathode mass and todistribute current to it.

Another aspect of the invention is an aluminium production cell of thetype with a drained cathode, wherein a cathode holder of electricallyconductive material is placed between an outer shell of the cell and thedrained cathode. This cathode holder is connected by collector bars tothe outside of the outer shell, whereby the cathode holder maintains thecollector bars at practically constant electrical potential leading to aconstant current distribution in the collector bars and a uniformdistribution of electric current in the cathode. This furthermoreeliminates current fluctuations due to poor distribution and flow ofcurrent typical in conventional cells, thereby reducing or eliminatingthe resulting non-uniform electro-magnetic field that can createmovement in the molten aluminium.

The cathode and its holder shell (or plate) are separated from the outershell of the cell by insulating and refractory materials such as theusual types of insulating bricks used for cell linings. It is alsopossible to provide an air or gas space between the inner shell and theinsulating and refractory materials. This space can be used to controlthe temperature of the inner shell by supplying heating or cooling gas,notably hot gas to heat the inner shell and cathode mass during cellstart up.

The cathode mass can be made mainly of carbonaceous material, such ascompacted powdered carbon, a carbon-based paste for example as describedin U.S. Pat. No. 5,362,366 (Sekhar et al), prebaked carbon blocksassembled together on the shell, or graphite blocks, plates or tiles.

It is also possible for the cathode to be made mainly of anelectrically-conductive non-carbon material, or of a composite materialmade of an electrically-conductive material and an electricallynon-conductive material.

In such a composite material, the non-conductive material can bealumina, cryolite, or other refractory oxides, nitrides, carbides orcombinations thereof and the conductive material can be at least onemetal from Groups IIA, IIB, IIIA, IIIB, IVB, VB and the Lanthanideseries of the Periodic Table, in particular aluminium, titanium, zinc,magnesium, niobium, yttrium or cerium, and alloys and intermetalliccompounds thereof.

The composite material's metal preferably has a melting point from 650°C. to 970° C., or above.

The composite material is advantageously a mass made of alumina andaluminium or an aluminium alloy, see U.S. Pat. No. 4,650,552 (de Nora etal), or a mass made of alumina, titanium diboride and aluminium or analuminium alloy.

The composite material can also be obtained by micropyretic reactionsuch as that utilizing, as reactants, TiO₂, B₂O₃ and Al.

The cathode can also be made of a combination of at least two materialsfrom : at least one carbonaceous material as mentioned above; at leastone electrically conductive non-carbon material; and at least onecomposite material of an electrically conductive material and anelectrically non-conductive material, as mentioned above.

The cathode should be impervious and resistant or substantiallyimpervious and resistant to molten aluminium and to the moltenelectrolyte, and can be rendered aluminium-impervious by one or morelayers of fibers and/or by layers of a composite material as discussedabove.

The cathode can comprise active cathode material and reinforcingmaterial, one example being carbon fibers impregnated with a slurry oftitanium diboride, possibly further impregnated with aluminium. It canalso comprise layers of imbricated tiles or slabs of carbon, anelectrically conductive material, or a composite material made ofelectrically conducting material and electrically non-conductingmaterial. Advantageously a cloth of aluminium impervious material isplaced between some or all of the layers of tiles or slabs.

The cathode most preferably has an upper active surface which isaluminium-wettable, for example the upper surface of the cathode iscoated with a coating of refractory aluminium wettable material asdescribed in U.S. Pat. No. 5,364,513 (Sekhar et al) and U.S. Pat. No.5,651,874 (Sekhar et al). Also, the upper surface of the inner shell incontact with the cathode can be coated with a coating of refractoryaluminium-wettable material or other protective materials.

The aluminium-wettable surface usually comprises a refractory boride,advantageously applied as a coating from a slurry of particles of therefractory boride or other aluminium-wettable material.

The aluminium-wettable surface can be obtained by applying a top layerof refractory aluminium-wettable material over the upper active surfaceof the cathode (which can already have a precoating of the refractoryaluminium wettable material) and over parts of the cell surrounding thecathode.

In most preferred embodiments, the cathode is a drained cathode.Preferably, the upper surface of the cathode is at a slope so as tooperate as a drained cathode, the upper surface of the cathode forexample comprising opposed sloping surfaces leading down into a centralchannel for the continuous removal of product aluminium. This centraldraining channel (or a side channel or several channels in otherembodiments) leads into an aluminium storage sump or space which isinternal or external to the cell and from which the aluminium can betapped from time to time, as described for instance in U.S. Pat. No.5,683,559 (de Nora).

Alternatively, the upper surface of the cathode comprises a series ofoppositely sloping surfaces forming therebetween recesses or channels ofvarious shapes, for example generally V-shaped.

The cathode current collector bars can either extend down through thebottom of the cell or extend out through the sides of the cell. In theformer case, each cathode comprises a plurality of cathode currentconnector bars extending down through the bottom of the cell, thecurrent connector bars being spaced apart along the center line of thecathode or being symmetrically distributed.

The cathode holder shell (or plate) is preferably made of metal or othersuitable highly electrically conductive material. Conveniently, thecathode holder shell is made of metal and comprises a substantially flatbottom with upwardly-protruding side edges approximately at right anglesto the substantially flat bottom or angled out relative to thesubstantially flat bottom. These upwardly-protruding edges can haveoutwardly projecting flanges that rest on shoulders of the cell sidewall. Such flanges can also be arranged to assist lifting of the entirecathode by a crane if desired for refurbishing.

The cathode holder shell's upwardly-protruding edges can extend allaround the periphery of the shell, but in some embodiments can extendonly partly around the periphery, for example along two opposite sides.In the case where a supporting plate is used, there are no upwardlyprotruding edges.

The cathode holder shell (or plate) is usually made of a sheet ofimperforate metal but can also be made of a sheet of perforated metal orof a series of metal members assembled together with or without spacingsbetween them, the arrangement being such that this shell fulfills itsfunction of supporting the cathode mass and uniformly distributingcurrent to the cathode mass.

It can also be made of a series of containers each having one or moreelectrical feeders.

Each cell can comprise a single cathode made up of a cathode supportedon its holder shell provided with current collector bars. In this case,the single cathode fits as a unit in a corresponding central recess inthe cell, and the cathode surface (usually drained) cooperates with aseries of anodes. For example, the cathode has a series of slopingdrained cathode surfaces facing corresponding sloping anode surfaces.

Alternatively, a cell design is contemplated where the cell bottom hasseveral recesses receiving a corresponding number of individualcathodes, each cathode cooperating with one anode or a series of anodes.In this case, the individual cathodes (inner cathode holder shell,cathode mass and current collector bar(s)) can each be installed andremoved as a unit.

The cells according to the invention can make use of traditionalconsumable prebaked carbon anodes, continuously-fed Söderberg-typeanodes, as well as non-consumable or substantially non-consumableanodes, such as metal anodes based on nickel-iron-aluminium ornickel-iron-aluminium-copper with an oxide surface, for example asdescribed in U.S. Pat. No. 5,510,008 (de Nora et al).

Whether consumable prebaked anodes or non-consumable anodes are used, itis advantageous to preheat each anode before it is installed in the cellduring operation, in replacement of a carbon anode which has beensubstantially consumed, or a non-consumable anode that has becomedisactivated or requires servicing. By preheating the anodes,disturbances in cell operation due to local cooling are avoided as whenan electrolyte crust is formed whereby part of the anode is not activeuntil the electrolyte crust has melted.

Another aspect of the invention is a cathode for a cell for theproduction of aluminium by the electrolysis of an aluminium compounddissolved in a molten electrolyte. This cathode comprises a cathode massformed mainly of electrically conductive material and a cathode holdershell (or plate) of good electrically conductive material such as metal.The cathode mass is supported on and substantially coextensive with theholder shell. An active cathode surface, such as a slurry-appliedcoating of an aluminium-wettable boride, is arranged on the uppersurface of the cathode mass which can itself be aluminium wettable, anda current collector bar is connected to the underside or sides of theholder shell for the supply of current to the cathode. This cathodeholder shell thus serves to feed current and uniformize distribution ofthe current supplied via the collector bar to the cathode mass.

This cathode can incorporate all of the features described above inrelation to the cell.

The invention also concerns a method of manufacturing this cathode,comprising providing a holder shell (or plate) made of one or moresheets or members of highly electrically-conductive material such asmetal, supporting on the cathode holder shell a cathode mass which issubstantially coextensive therewith to form a cathode mechanicallysupported by and electrically connected to the holder shell, andconnecting at least one current collector bar to the underside of theholder shell, or to its side(s).

Another inventive aspect is a method of producing a cathode andinstalling it as a unit in an aluminium production cell, the same methodapplying equally to producing and installing a series of cathodes. Thismethod comprises placing an electrically-conductive cathode mass (forexample mainly of carbonaceous material) on a cathode holder shell (orplate) to form a cathode wherein current can be supplied to the cathodemass by a current collector bar and distributed uniformly over thecathode mass by the holder shell. This cathode, comprising the cathodemass placed on its holder shell, is then installed in an outer shellforming the bottom and sides of the cell, and the inner cathode holdershell is connected to the outside of the outer shell by a currentcollector bar.

The invention also provides an improved cathode pot of a cell for theproduction of aluminium by the electrolysis of an aluminium compounddissolved in a molten electrolyte, of the type comprising an outermechanical structure which forms an outer shell of the cathode pot, andan electric and thermic insulator forming a potlining. In known cells, acathode is supported on the electric and thermic insulator whichseparates the cathode from the outer shell, and at least one conductorbar connects the cathode to outside the outer shell for connection to anexternal negative busbar, the or each conductor bar extending throughthe electric and thermic insulator.

The improved cathode pot according to the invention includes at leastone cathode which advantageously can be installed in and removed fromthe cathode pot as a unit. The or each cathode comprises a cathodeholder formed by a metallic shell or plate of an electrically conductingmaterial and a cathode mass constituted mainly ofelectrically-conductive material supported by the cathode holder, thecathode mass preferably having an aluminium-wettable active surface. Theor each cathode holder is connected to outside the outer shell by atleast one said conductor bar, the cathode holder serving to uniformizedistribution of electric current from the conductor bars(s) to thecathode mass.

The invention also pertains to a method of supplying electric current toa cathode mass of an aluminium production cell, the method comprisingsupplying current via one or more cathode current collector bars to thebottom of the cathode mass, the current collector bar(s) being of smallcross-section compared to the size of the cathode bottom. The currentsupplied via the current collector bar is distributed uniformly over theentire bottom of the cathode mass by means of a current distributorshell (or plate) substantially coextensive with the entire bottom of thecathode mass, thus serving to keep the entire bottom of the cathode atpractically the same potential. The current passing from the currentdistributor shell into the cathode mass is hence evenly distributed overthe cathode mass. Moreover, when several current collector bars areconnected to the current distributor shell, the current collector barsare held at the same potential which equalizes current supply via thecollector bars.

The invention also provides for renovating an aluminium production cellaccording to the invention after the cell has been taken out of service.This method comprises also the possibility of removing, as a unit, theor each used cathode and its support shell and replacing each entireused cathode by one or more new or renovated cathode units. By thismeans, renovation of the cell is greatly simplified because removal ofthe cathode as one or more units avoids the need to mechanically breakup the used cathode mass using jackhammers or like tools, which hasheretofore been the usual practice. Furthermore, installing the new orrenovated cathode is much simpler than rebuilding a new cathode liningin situ.

The invention also contemplates transforming an existing Hall-Héroultcell into a cell according to the invention by shutting down the celland removing the used cathode for example in the normal way usingjackhammers, refurbishing and/or rebuilding the insulating lining formedby the electric and thermic insulating mass as necessary, and fittingone or more cathode units as discussed above.

A method of producing aluminium according to the invention using thecell as outlined above, involves supplying current to the cathode viathe current collector bar and the or each cathode holder shell (orplate) which distributes the current to the cathode mass evenly andmaintains the cathode current collectors at the same potential. As aresult, in cell operation, there are less disturbances byelectromagnetic fields due to horizontal electric currents in the metal,and the overall cell efficiency is improved.

Advantageously, the surface of the cathode mass is maintained at atemperature corresponding to a paste state of the electrolyte wherebythe cathode mass is protected from chemical attack. For example, whenthe cryolite-based electrolyte is at about 950° C., the surface of thecathode mass can be cooled by about 30° C., whereby the electrolytecontacting the cathode surface forms a viscous paste which protects thecathode surface. The surface of the cathode mass can be maintained atthe selected temperature by supplying gas via an air or gas spacebetween the cathode holder and the electric and thermic insulating mass.

The cathodes of the invention can also be used in a novel arrangementfor conducting electric current between aluminium electrowinning cellsdisposed in side by side relationship wherein the busbar connected tothe inner cathode holder shell of the cathode of one cell is connecteddirectly to the anode current supply of an adjacent cell.

In such an arrangement, each cell comprises a cell base having acathodic cell bottom fitted with current collector bars at or adjacentto the bottom of the cell for feeding current to the cathodic cellbottom, and a cell superstructure comprising anodes and means forsupplying current to the anodes. The cells can be connected so thatcurrent is conducted between the adjacent cells by conductor barscrossing-over from one cell to an adjacent cell, each crossing-overconducting bar connecting at least one anode at the top of one sectionof one cell to at least one corresponding conductor bar at or adjacentto the bottom of a corresponding section of the adjacent cell. Thisconductor bar is advantageously connected to the inner cathode holdershell of a cathode of a cell according to the invention, as describedabove.

In this arrangement, the anodes in each cell can be arranged in two rowsof side-by-side anodes with pairs of side-by-side anodes in the two rowsconnected together, and with each crossing-over conductor bar connectedto at least one pair of interconnected anodes. For example, eachcrossing-over conductor bar is connected to two adjacent pairs ofinterconnected anodes.

Advantageously, the cells are arranged side-by-side in rows, the pairsof cells in each row being connected in parallel to corresponding pairsof cells in the adjacent rows. Moreover, each crossing-over conductorbar can be connected to at least two cross-wise current collector barsin the cell bottom.

This new arrangement has pairs of cells connected in parallel, havingthe advantage that each cell can be smaller and more efficient.Moreover, the total voltage of a cell line is consequentlyadvantageously lower.

The invention also pertains to a system of interconnected cells for theproduction of aluminium by the electrolysis of an aluminium compounddissolved in a molten electrolyte, advantageously cells including theimproved cathodes as defined above, wherein each cell comprises an anodesuspension and current-supply superstructure and a cathode cell bottomassociated with cathode current supply means.

The cells making up this system are arranged in rows, each row beingmade up of an alignment of pairs of side-by-side cells. The anodecurrent-supply superstructures of the two cells of each side-by-sidepair of cells of one row are connected together to a common anodebusbar. The cathode current supply means of the two cells of eachside-by-side pair of cells of one row are connected together and then tothe common anode busbar of a corresponding side-by-side pair of cells ofan adjacent row of cells.

In this manner, corresponding pairs of side-by-side cells in the rows ofcells are connected together in parallel, leading to a simplification ofthe buswork compared to conventional arrangements. Moreover, connectionof the cells in parallel doubles the current capacity and enables cellsto be cut-off one at a time to allow maintenance operations on theoff-circuit cells. As discussed above, each parallel-connected cell canmade be smaller and more efficient, and the total voltage of a cell linereduced.

Preferably, the cells of each side-by-side pair of cells of one row areplaced close together with their common anode busbar situated betweenthem, and the cells of adjacent rows are spaced apart from one anotherleaving between them a walkway allowing access to all of the cells forservicing. This arrangement permits access to all cells with a reducedspace for walkways, namely half as many are needed compared toconventional arrangements with walkways along both sides of the cells.

In this arrangement, the cathode current supply means preferablycomprises a current collector bar that projects vertically downwardsfrom the bottom of each cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to theaccompanying schematic drawings, in which :

FIG. 1 is a cross-sectional view of one aluminium production cellaccording to the invention;

FIG. 2 is a cross-sectional view of another aluminium production cellaccording to the invention;

FIG. 3 shows the bottom part of the cell of FIG. 2 during assembly of acathode unit;

FIG. 4 shows in longitudinal cross-section an embodiment of the cathodeready to be installed in a cell;

FIG. 5 is a longitudinal cross-sectional view of another aluminiumproduction cell according to the invention;

FIGS. 6 and 7 are cross-sectional views of further aluminium productioncells according to the invention;

FIG. 8 is a cross-sectional view through part of another embodiment ofthe aluminium production cell according to the invention;

FIG. 8a is plan view of the cathode pot of the cell of FIG. 8 duringconstruction;

FIG. 8b is a cross-section along line b—b of FIG. 8a;

FIG. 9 is a schematic cross-section through a system of interconnectedaluminium production cells according to the invention, wherein theadjacent cells of different rows are connected cross-wise in series; and

FIG. 10 is a schematic cross-section through another system ofinterconnected aluminium production cells according to the invention,wherein pairs of adjacent cells of different rows are connectedcross-wise in parallel.

DETAILED DESCRIPTION

FIG. 1 schematically shows an aluminium production cell according to theinvention wherein a plurality of anodes 10 are suspended by yokes 11connected to an anode suspension and current supply superstructure (seefor example FIGS. 9 and 10) which hold the anodes 10 suspended above acathode cell bottom 20 enclosed in an outer steel shell 21 forming, withits insulating lining of refractory bricks 40, a cell trough or cathodepot.

Inside the outer steel shell 21 is housed a cathode 30 comprising aninner steel cathode holder shell 31 containing a cathode mass 32. Asillustrated, the inner shell 31 has a flat bottom, side walls 33 andoutwardly-directed side flanges 34 at its top. The inner shell 31 formsan open-topped container for the cathode mass 32.

The cathode mass 32 can for example be made of packed carbon powder,graphitized carbon, or stacked plates or slabs of carbon imbricated withone another and separated by layers of a material that is impermeable tothe penetration of molten aluminium. Alternatively, the cathode mass canbe made mainly of other electrically conductive materials or compositematerials, as discussed above.

The top of the cathode 32 mass has inclined surfaces 35 leading downinto a central channel 36 for draining molten aluminium. On top of thecathode mass 32, and also extending over the flanges 34, is a coating 37of aluminium-wettable material, preferably a slurry-applied boridecoating as described in U.S. Pat. No. 5,651,874 (Sekhar et al). Suchcoating 37 can also be applied to the inside surfaces of the bottom andsides 33 of the cathode holder shell 31, to improve electricalconnection between the inner shell 31 and the cathode mass 32.

In the example of FIG. 1, the cathode mass 32 does not protrude abovethe tops of the sidewalls 33 of shell 31. In this embodiment, theperiphery of the cathode mass 32 extends to the top of the sidewalls 33,from where it slopes down to the central channel 36.

The cathode 30 is supported as a removable unit in the cell bottom 20 ina central recess of corresponding shape in the refractory bricks 40lining the outer steel shell 21. These refractory bricks 40 are theusual types used for lining conventional cells.

Current is supplied to the cathode 30 via transverse conductor bars 41welded to the bottom of the inner shell 31. These conductor bars 41 areconnected to current collector bars 42 which protrude laterally from thesides of the outer shell 21, these collector bars 42 being connected toexternal buswork (not shown).

Alternatively, current could be supplied to the cathode 30 of FIG. 1, bya series of vertical current collector bars 42 extending down throughvertical openings in the bottom of the lining formed by the refractorybricks 40 (see FIG. 2).

Due to the metallic conductivity of the cathode holder shell 31, theseconductor bars 41 are all maintained at practically the same electricalpotential leading to uniform current distribution in the collector bars42. Moreover, the metal inner shell 31 evenly distributes the electriccurrent in the cathode mass 32.

Inside the part of the cell side walls at the top of the outer shell 21facing the sides of anodes 10 is a lining 50 formed for example ofplates of silicon carbide. Alternatively, the lining 50 could be made oftreated carbon coated with a slurry-applied coating of refractoryboride, like the coating 37.

The cathode 30 can be manufactured as a separate unit that can beinstalled in the cell bottom 20, composed of the outer steel shell 21lined with refractory bricks 40 and already fitted with the lateralcurrent collector bars 42 which are ready to be connected to thetransverse conductor bars 41 when the cathode 30 is installed. Thesilicon carbide plates 50 can be fitted before or after insertion of thecathode 30.

The cathode 30 can be produced by first forming the inner steel cathodeholder shell 31 with its side walls 33 and flanges 34, then applying aboride coating 37 to the inner surface of the shell 31 if desired. Thecathode mass 32 is then placed in the inner shell 31. The centralchannel 36 and sloping surfaces 35 can be preformed if the cathode mass32 is made of blocks, or can be formed by a shaping operation after thecathode mass is placed in the cathode holder shell 31, for example ifthe cathode mass 32 is made from a compacted powder or paste. One ormore coats of refractory boride coating 37 can then be applied to thetop of the cathode mass 32 by the application of a slurry, drying andbaking as required. Further coats of the refractory boride coating 37can be applied to the top of the cathode mass 32, to the flanges 34 andpossibly to a surrounding part of the refractory bricks 40 after thecathode 30 has been installed. The current conductor bars 41 can bewelded when the inner steel shell 31 is being or has been formed, beforethe cathode mass 32 has been put in place.

In use, the space between the cathode 30 and the side-wall lining 50 isfilled with a molten electrolyte such as cryolite containing dissolvedalumina at a temperature usually about 950-970° C., and into which theanodes 10 dip. When electrolysis current is passed, aluminium is formedon the sloping cathode surfaces 35 coated with the refractory boridecoating 37, and the produced aluminium continuously drains down thesloping surfaces 35 into the central channel 36 from where it is removedpermanently into an internal or external storage located usually at oneend of the cell.

The anodes 10, which are shown as being consumable prebaked carbonanodes, have sloping surfaces 12 facing the sloping cathode surfaces 35.The inclination of these anode surfaces 12 facilitates the release ofbubbles of the anodically-released gases. As the anode 10 is consumed,it maintains its shape, keeping a uniform anode-cathode spacing.Alternatively, it would be possible for the same cell bottom 20 and itscathode 30 to be used with non-consumable or substantiallynon-consumable anodes.

Periodically, when the cathode 30 needs servicing, it is possible toclose down the cell, remove the molten cell contents, and disassemblethe entire cathode 30 to replace it with a new or a serviced cathode 30.This operation is much more convenient and less labour intensive thanthe conventional cell bottom relining process, has reduced risksrelating to exposure to the toxic waste materials, and simplifiesdisposal of the toxic waste materials.

The aluminium production cell shown in FIG. 2 is similar to that of FIG.1 and like references have been used to designate like parts. In thisdesign, the current collector bars 42 instead of being horizontal arevertical and extend through vertical apertures 43 in the lining ofbricks 40. These collector bars 42 are welded centrally to the bottom ofthe inner shell 31. As illustrated in FIG. 4, several collector bars 42are spaced apart from one another along the bottom of the inner shell31. These collector bars 42 can have any desired cross-sectional shape :circular, rectangular, T-shaped, etc. Because the inner metal shell 31keeps the collector bars 42 at practically the same potential,fluctuations in the current supply are avoided.

The assembly method is illustrated in FIG. 3. It is possible to installthe entire cathode 30 by lowering it using a crane until the bottom ofthe cathode holder shell 31 comes to rest on the top 44 of the lining ofbricks 40 and its side flanges 34 come to rest on shoulders 45 of thecell lining. Then, the plates 50 of silicon carbide can be installed ontop of the flanges 34. This assembly method is simple and labour saving,compared to the usual cell lining methods used heretofore.

To dismantle the cell, the plates 50 are removed first, then the cathode30, after disconnecting the collector bars 42 from the negative busbar.This dismantling of the cell is remarkably simple to carry out andconsiderably simplifies disposal of toxic wastes.

FIG. 4 shows another embodiment of the cathode 30 ready to be installedas a unit in an aluminium production cell. This cathode comprises ametal cathode holder shell 31 made of a flat base plate to which sidewalls 33 are welded substantially at right angles along its side edges.These side walls 33 can extend around the entire periphery of the baseplate, or only along its opposite side edges.

To the bottom of the shell 31's base plate, a series of conductor bars42 are welded, spaced equally apart from one another along the length ofthe shell 31. These conductor bars 42 protrude vertically down from theshell 31, so they can pass through corresponding vertical openings inthe cell bottom, for connection to an external negative busbar.

In the shell 31 is a cathode mass 32 formed of a series of blocks, forexample of carbon. As shown, the cathode blocks have sloping uppersurfaces 35 and are fitted together to form a series of generallyV-shaped recesses. In this example, parts of the cathode blocks protrudeabove the top of the side walls 33 which are embedded in the sides ofthe end blocks.

The upper surface 35 is made up of a series of sloping surfaces ingenerally V-configuration, formed by placing the adjacent blockstogether. Each conductor bar 42 corresponds to the junction between twoadjacent blocks forming the lower part of each V. As shown, theconductor bars 42 protrude through the shell 31 and extend part of theway up the blocks 42. Alternatively, the conductor bars 42 could bewelded externally to the bottom of the shell 31.

Before use, the entire sloping upper surface 35 of the cathode mass 32is coated with an aluminium-wettable coating typically formed ofslurry-applied titanium diboride.

This cathode 30 can be produced as a unit and installed in an aluminiumproduction cell (as illustrated in FIGS. 3) by lifting it with a crane,and lowering it into the cell.

The aluminium production cell shown in longitudinal cross-section inFIG. 5 comprises a cathode 30 with a series of spaced-apart verticalcurrent conductors 42 welded to the bottom of its inner cathode holdershell 31, these conductors 42 protruding from the lower face of the cellbottom 20 for connection to the cathode buswork (see FIGS. 9 and 10).

The cathode mass 32 is made up of several layers of a conductivematerial such as carbon possibly combined with materials rendering thecarbon impervious to molten aluminium. The mass 32 comprises an outerlayer around the bottom and sides 33 of the inner shell 31. This outerlayer has a peripheral edge 32 a surrounding a central recess that iscoated with a flat layer 38 of carbon or other conductive material ontop of which is a top layer 39 having sloping faces 35 coated with thelayer 37 of aluminium-wettable boride. As illustrated, theupwardly-sloping side parts of the faces 35 are extended by bevelledparts of the edges 32 a and by ramming paste 51, forming wedges alongthe edges of the cathode mass 32.

The sloping faces 35 of the top layer 39 are inclined alternately toform flattened V-shaped recesses above which the anodes 10 are suspendedwith corresponding V-shaped inclined faces 11 of the anodes facing theV-shaped recesses in the cathode 30. The anodes 10 are suspended bysteel rods 14 held at an adjustable height in attachments 15 by an anodebus 16, enabling the anodes 10 to be suspended with a selectedanode-cathode gap.

Assembly and disassembly of the cathode 30 of this cell is similar towhat has been described previously. However, assembly of the layersmaking up the cathode mass 32 will be different. Its outer layer withedge 32 a can be made of carbon blocks or compacted powder. The flatlayer 38 can be compacted powder or layers of carbon tiles or platesintegrating layers of an aluminium-impervious material, and the shapedtoplayer 39 can be made of preformed graphitized carbon blocks. Allthese layers can be bonded by a conductive paste or adhesive, inparticular a boride-based paste as described in U.S. Pat. No. 5,320,717(Sekhar). Alternatively, the layered cathode mass 32 can be made mainlyof an electrically-conductive non-carbon material, aconductive/non-conductive composite, or alternating carbon/non-carbonlayers.

The cathode 30 is assembled first, outside the cell, then lowered usinga crane into the cell bottom 20, passing the conductor bars 42 throughcorresponding openings 43 in the bricks 40. Then the gaps around theedges of the cathode mass 32 are filled with ramming paste 51 which isformed into the side wedges. Next, a slurry of refractory boride isapplied to the sloping cathode faces 35, usually on top of a pre-coatingalready applied thereto, and also over the sloping wedge surfaces of theedges 32 a and ramming paste 51. After drying and heat treatment of theboride coating 37, the cell is ready for start-up. In operation, thecentral recess in the cell above the cathode mass 32 contains a moltenelectrolyte, such as cryolite containing dissolved alumina, into whichthe anodes 10 dip.

For disassembly to service the cell bottom 20, the molten contents areremoved from the cell, and the ramming paste 51 is broken to enable theentire cathode unit 30 to be lifted out of the cell using a crane, afterhaving disconnected the conductor bars 42 from the cathode busbar.

FIG. 6 shows a modified cell wherein the bottom of the cathode holdershell 31 is held spaced apart above the top of the refractory bricks 40by girders 51, to leave therebetween an air or gas space 52 which actsas a thermic insulating space. Also, it is possible to adjust thetemperature of the cathode 30 (shell 31 and cathode mass 32) bysupplying a heating or cooling gas to the space 52. For example, duringcell start up, the cathode 30 can be heated by passing hot gas throughspace 52. Or during operation, the surface of the cathode mass 32 can becooled to make the electrolyte contacting it form a protective paste.

Also illustrated in FIG. 6 is a varied design where extra plates 53 ofsilicon carbide or treated carbon coated with a slurry-applied coatingof refractory boride, like the coating 37, are placed in the cell liningso as to fit against the side walls 33 of shell 31 when the cathode 30is installed.

Also shown in FIG. 6 is the molten electrolyte 54, a crust 56 ofsolidified electrolyte, and molten product aluminium 57 in the channel36.

In the embodiment of FIG. 6, the facing surfaces of the cathode 30 andanode 10 are shown as flat. However, it is understood that these surfacecan be sloping when seen in longitudinal cross-section, as shown in FIG.4.

FIG. 7 illustrates a cell wherein the cathode mass 32 is supported by acathode holder plate 31′ resting on girders 51 which provide an air orgas space 52, as in FIG. 6. This cathode holder plate 31′ is generallyflat but has a central recess corresponding to the location of thecentral channel 36 which receives the drained molten aluminium 53. Thisrecessed central part of the cathode holder plate 31′ corresponds inthickness to the girders 51, and rests on the top layer of bricks 40,like the girders 51. The current collector bars 42 are welded to thebottom of this recessed central part of the cathode holder plate 31′.The central recess 36 extends down to about the level of the main partof plate 31′ and is narrower than the corresponding central recess inplate 31′. The material of cathode mass 32 thus extends substantiallyall over the plate 31′ including its recessed part. However, the sidesof the cathode mass 32 stop short of the edges of the cathode holderplate 31′, leaving a space to receive the silicon carbide plates formingthe lining 50.

The cathode mass 32 is advantageously a compositealumina-aluminium-titanium diboride material, for example produced bymicropyretic reaction of TiO₂, B₂O₃ and Al. Such composite materialsexhibit a certain plasticity at the cell operating temperature; whensupported by a rigid cathode holder plate 31′ or shell 31, thesematerials have the advantage that they can accommodate for thermaldifferences during cell start up and operation, while maintaining goodconductivity required to effectively operate as cathode mass.

The top surface of the cathode mass 32 is horizontal or very slightlyinclined and is coated with a slurry-applied layer of titanium diboride,forming a drained cathode surface 37.

Above the drained cathode surfaces 37 are suspended non-carbon oxygenevolving anodes 10′ fitted under a current distribution structure 10″attached at the lower end of vertical current supply bars 14. Theseanodes 10′ are advantageously metal anodes based on nickel-iron-aluminumor nickel-iron-aluminum-copper with an oxide surface, for example asdescribed in U.S. Pat. No. 5,510,008 (de Nora et al), possibly protectedin use by an in-situ formed cerium oxyfluoride coating as described inU.S. Pat. No. 4,614,569 (Duruz et al).

The top of this cell is enclosed by covers 58 which can be opened toallow access for servicing the anodes 10′. FIG. 7 also shows a crustbreaker 60 which can be lowered between the rows of anodes 10′ to breakthe crust formed on top of electrolyte 54. At the same location, butoffset longitudinally, are point feeders for supplying alumina toreplenish the electrolyte 54 in the central recess 36.

FIG. 8 illustrates part of a cell comprising a cathode holder made up ofseveral plates, seen in a cross section through one of the cathodeholder plates 31 a, out of the plane of the current collector bars 42(see FIG. 8b). The cathode pot 20 of this cell is assembled by placing aseries of rectangular steel cathode holder plates 31 a, each with twocurrent collector bars 42, onto the lining of bricks 40. The adjacentcathode holder plates 31 a are spaced apart and rest on girders 61 ofinverted T shape resting on the top of bricks 40. Around the sides ofthe cathode pot 20 are lining plates 50 of silicon carbide or treatedcarbon, forming a shell all around the cathode pot. Inside this shell,the protruding parts of girders 61 form sub-divisions. This shell isfilled with an electrically-conductive cathode mass 32, advantageouslymade of a composite material containing aluminium, alumina and possiblytitanium diboride, and coated with an aluminium-wettable titaniumdiboride coating 37. This cathode mass 32 can fill the space behind thelining 50, as shown in FIG. 8.

FIGS. 8, 8 a and 8 b thus illustrate an embodiment of the inventionwherein the cathode holder is made up of a plurality of plates 31 aspaced from one another with the girders 61 bridging the spaces. In onevariation of this embodiment, each individual plate 31 a could alreadycarry a cathode mass, the gaps between the masses of the adjacent platesbeing filled with a suitable paste or powder mix. In another variationof this embodiment, each individual plate 31 a be replaced by anindividual cathode shell containing a cathode mass whereby the cellincludes several cathode holder shells.

FIG. 9 shows three cells of a series of aluminium production cellsincorporating cathodes 30 as described previously, and disposed inside-by-side rows. Each cell comprises a cell base 20 forming a cathodiccell bottom having current collector bars 42 leading in to the bottom ofthe cell for feeding current to the cathode mass 32 via the innercathode holder shell 31. The cell superstructure comprises anodes 10suspended in pairs from yokes 11, a vertical iron bar 14 and attachments15 connected to an anode bus 16 forming means for supplying current tothe anodes 10.

Each cell also has a fume cover 58 that is removable or has removableparts to permit replacement of the anodes 10 when needed, and for theperiodic supply of alumina to replenish the molten electrolyte.

The adjacent cells are connected so that current is conducted betweenthem by conductor bars 17 crossing-over from one cell to an adjacentcell. As illustrated, the conductors 17 are extended by flexiblealuminium sheets 18 connected to the anode bus 16 and attachment 15.Each crossing-over conducting bar 17 connects the anodes 10 at the topof one section of one cell to at least one corresponding currentconductor bar 42 at the bottom of a corresponding section of theadjacent cell. Such conductor bar 42 is advantageously connected to theinner cathode holder shell 31 of a cathode 30 as described above.

Between each adjacent side-by-side pair of cells is a walkway 55adjacent to the top of the cell trough, these walkways 55 allowingworkmen to access the cells to service them.

In the illustrated arrangement, the anodes 10 in each cell are arrangedin two rows of side-by-side anodes 10 with pairs of side-by-side anodesin the two rows connected together by the yokes 11. Each crossing-overconductor bar 17 is connected via the aluminium sheets 18 andattachments 15 to at least one pair of interconnected anodes 10.

Each crossing-over conductor bar 17 can be connected to one or morecorresponding current collector bars 42 in the cell bottom.

FIG. 10 shows part of a system of interconnected aluminium productioncells including the improved cathodes 30 as described above. Each cellhas an anode suspension and current-supply superstructure 11, 14, 15 anda cathode cell bottom 20 associated with cathode current supply meansformed by vertical current collector bars 42 and cathode holder shells31.

The cells making up this system are arranged in rows, each row beingmade up of an alignment of pairs of side-by-side cells. FIG. 10 showsthree rows of cells in side-by-side pairs. However, any convenientnumber of rows of cells can be arranged across the cellroom, each rowbeing made up of a convenient number of pairs of side-by-side cells.

As shown, the anode current-supply superstructures 11, 14, 15 of the twocells of each side-by-side pair of cells of one row are connectedtogether to a common central anode busbar 19 by flexible aluminiumsheets 18.

The cathode current collector bars 42 of the two cells of eachside-by-side pair of cells of one row are connected together and then tothe common anode busbar 19 of a corresponding side-by-side pair of cellsof an adjacent row of cells by the conductors 17 and flexible aluminiumsheets 18.

In this manner, corresponding pairs of side-by-side cells in the rows ofcells are connected together in parallel, leading to a simplification ofthe buswork compared to conventional arrangements. Connection of thecells in parallel doubles the current capacity of the cellroom andenables cells to be cut-off one at a time to allow maintenanceoperations on the off-circuit cells. This also has the advantage thateach cell can be smaller and more efficient. Moreover, the total voltageof a cell line is consequently advantageously lower.

As illustrated, the cells of each side-by-side pair of cells of one roware placed close together with their common anode busbar 19 situatedbetween them, and the cells of adjacent rows are spaced apart from oneanother leaving space for a walkway 55 allowing access to all of thecells for servicing. This arrangement permits access to all cells with areduced space for walkways 55, namely half as many are needed comparedto conventional arrangements (and the arrangement shown in FIG. 9) whichhave walkways along both sides of the cells.

What is claim is:
 1. A cell for the production of aluminium by theelectrolysis of an aluminium compound dissolved in a molten electrolyte,comprising an outer mechanical structure forming an outer shell one ormore cathodes and an electric and thermic insulation separating the oreach cathode from the outer shell, the outer shell and the electric andthermic insulation forming a recess that houses the or each cathode, theor each cathode comprising an inner electrically-conductive cathodeholder supporting and substantially coextensive with a cathode mass, thecathode holder being connected electrically to a busbar, the or eachcathode holder also serving to distribute current to its cathode mass,wherein the or each cathode holder and the thereon supported cathodemass are movable as an individual cathode unit within said recess forinsertion therein and removal therefrom of said individual cathode unit.2. The aluminium production cell of claim 1, wherein the cathode masshas an aluminium-wettable surface.
 3. The aluminium production cell ofclaim 2, wherein the cathode is a drained cathode.
 4. The aluminiumproduction cell of claim 3, wherein the upper surface of the cathodemass comprises at least one drained surface which is at a slope.
 5. Thealuminium production cell of claim 4, wherein the upper surface of thecathode mass comprises opposed sloping surfaces leading down into acentral channel for the removal of product aluminium.
 6. The aluminiumproduction cell of claim 4, wherein the upper surface of the cathodemass comprises a series of oppositely sloping surfaces formingtherebetween a series of recesses or channels of any shape, preferablygenerally V-shaped.
 7. The aluminium production cell of claim 2, whereinthe cathode mass is made mainly of carbonaceous aluminum wettablematerial.
 8. The aluminium production cell of claim 7, wherein thecarbonaceous material comprises compacted powdered carbon or carbonpaste.
 9. The aluminium production cell of claim 8, wherein thecarbonaceous material comprises prebaked carbon blocks.
 10. Thealuminium production cell of claim 7, wherein the cathode mass comprisesgraphite blocks, plates or tiles.
 11. The aluminium production cell ofclaim 1, wherein the cathode mass is made mainly of an electricallyconductive non-carbon material.
 12. The aluminium production cell ofclaim 11, wherein the cathode mass is made of a composite material madeof an electrically conductive material and an electricallynon-conductive material.
 13. The aluminium production cell of claim 12,wherein the non-conductive material is alumina, cryolite, or otherrefractory oxides, nitrides, carbides or combinations thereof.
 14. Thealuminium production cell of claim 13, wherein the conductive materialcontains at least one metal from aluminium, titanium, zinc, magnesium,niobium, yttrium or cerium, and alloys and intermetallic compoundsthereof.
 15. The aluminium production cell of claim 12, wherein thecomposite material is a mass comprising alumina with aluminium or analuminium alloy.
 16. The aluminium production cell of claim 15, whereinthe composite material is a mass made of alumina, titanium diboride andaluminium.
 17. The aluminium production cell of claim 16, wherein thecomposite material is obtained by reaction in which the reactants areTiO₂, B₂O₃ and Al.
 18. The aluminium production cell of claim 12,wherein the conductive material contains at least one metal from GroupsIIA, IIB, IIIA, IIIB, IVB, VB and the Lanthanide series of the PeriodicTable, and alloys and intermetallic compounds thereof.
 19. The aluminiumproduction cell of claim 18, wherein the metal has a melting point from650° C. to 970° C.
 20. The aluminium production cell of claim 1, whereinthe cathode mass is substantially resistant and impervious to moltenaluminium and to the molten electrolyte.
 21. The aluminium productioncell of claim 1, wherein the cathode mass comprises active cathodematerial and reinforcing material.
 22. The aluminium production cell ofclaim 1, wherein the cathode mass comprises layers of imbricated tilesor slabs of: carbon, an electrically conductive material, or a compositematerial made of electrically conductive material and electricallynon-conductive material.
 23. The aluminium production cell of claim 22,wherein the cathode mass comprises a cloth of aluminium-imperviousmaterial between the layers of tiles or slabs.
 24. The aluminiumproduction cell of claim 1, wherein the cathode holder is a metallicshell having upwardly-protruding side edges.
 25. The aluminiumproduction cell of claim 24, wherein the metallic cathode holder shellhas a substantially flat bottom from which the upwardly-protruding sideedges are angled out, or are substantially at right angles, or areangled inwardly relative to the substantially flat bottom.
 26. Thealuminium production cell of claim 24, wherein the side edges of thecathode holder shell have outwardly projecting flanges.
 27. Thealuminium production cell of claim 1, wherein the cathode holder has acurved bottom or a generally V-shaped bottom in cross section.
 28. Thealuminium production cell of claim 1, wherein the cathode holder is madeof a sheet of imperforate metal.
 29. The aluminium production cell ofclaim 1, wherein the cathode holder is made of a sheet of perforatedmetal.
 30. The aluminium production cell of claim 1, wherein the cathodeholder is made of a plurality of metal members with or without spacingsbetween the members.
 31. The aluminium production cell of claim 1,wherein the top of the cathode mass comprises parts which protrude abovethe sides of the cathode holder.
 32. The aluminium production cell ofclaim 1, wherein the top of the cathode mass does not extend above thesides of the cathode holder.
 33. The aluminium production cell of claim1, wherein the cathode holder is connected to the outside of the outershell by a plurality of current collector bars, the cathode holdermaintaining the collector bars at practically the same electricalpotential to provide a constant current distribution in the collectorbars.
 34. The aluminium production cell of claim 33, wherein the cathodecurrent collector bars extend down through the bottom of the cell. 35.The aluminium production cell of claim 34, wherein the current collectorbars are spaced apart along the center line of the cathode holder or aresymmetrically distributed.
 36. The aluminium production cell of claim33, wherein the cathode current collector bars extend out through thesides of the cell.
 37. The aluminium production cell of claim 1, whereinthe upper surface of the cathode mass is coated with a coating ofrefractory aluminium-wettable material.
 38. The aluminium productioncell of claim 1, wherein the upper surface of the cathode holder incontact with the cathode mass is coated with a layer of refractoryaluminium-wettable material.
 39. The aluminium production cell of claim1, comprising at least one aluminium-wettable surface that comprises arefractory boride.
 40. The aluminium production cell of claim 1,comprising an aluminium-wettable coating applied from a slurry ofparticles of aluminium-wettable material.
 41. The aluminium productioncell of claim 40, comprising an aluminium-wettable surface obtained byapplying a top layer of refractory aluminium wettable material over theupper surface of the cathode mass and over parts of the cell surroundingthe cathode mass and in contact with the electrolyte.
 42. The aluminiumproduction cell of claim 1, wherein the top of the cathode masscomprises bodies such as tiles or blocks made of or coated with analuminium-wettable electrically-conductive material.
 43. The aluminiumproduction cell of claim 42, wherein said bodies protrude upwardly froma cathode mass made of an electrically-conductive material.
 44. Thealuminium production cell of claim 43, wherein the cathode mass iscoated with an aluminium-wettable material.
 45. The aluminium productioncell of claim 1, wherein the cathode holder(s) supporting the cathodemass is/are removably mounted in the outer shell of the cell.
 46. Thealuminium production cell of claim 45, wherein the current collectorbars are fixed to the bottom of the removable cathode holder, thecurrent collector bars extending down though openings in the electricand thermic insulation and through the bottom of the outer shell of thecell.
 47. The aluminium production cell of claim 1, wherein an air orgas space is provided between the cathode holder and the electric andthermic insulation.
 48. A cathode unit for a cell as defined in claim 1which cell has a recess for insertion therein and removal therefrom ofsaid individual cathode unit, the cathode unit comprising an innerelectrically-conductive cathode holder supporting and substantiallycoextensive with a cathode mass, the cathode holder being arranged forelectrical connection to a busbar, the or each cathode holder(s) alsoserving to distribute current to its cathode mass, wherein the cathodeholder and the theron supported cathode mass forming said individualcathode unit which is movable within said call recess for insertiontherein and removal therefrom of said individual unit.
 49. The cathodeunit of claim 48, wherein the cathode holder is a metallic shell havingupwardly-protruding side edges.
 50. The cathode unit of claim 49,wherein the cathode holder shell has a substantially flat bottom fromwhich the side edges are angled out, are substantially at right angles,or are angled inwardly relative to the substantially flat bottom. 51.The cathode unit of claim 49, wherein the cathode holder is a shell orplate made of a sheet of perforated metal.
 52. The cathode unit of claim48, wherein the upwardly-protruding edges have outwardly projectingflanges.
 53. The cathode unit of claim 48, wherein the cathode holderhas a curved bottom or a generally V-shaped bottom in cross section. 54.The cathode unit of claim 48, comprising a plurality of spaced apartcurrent collector bars connected at approximately right angles to thebottom of the cathode holder shell or plate.
 55. The cathode unit ofclaim 54, wherein the current collector bars are spaced apart along thecenter line of the cathode holder or are symmetrically distributed. 56.The cathode unit of claim 48, wherein the cathode current collector barsextend out of the sides of the cathode.
 57. The cathode unit of claim48, wherein the cathode holder is a shell or plate made of a sheet ofimperforate metal.
 58. The cathode unit of claim 48, wherein the cathodeholder is a shell or plate made of a plurality of metal members with orwithout spacings between the members.
 59. The cathode unit of claim 48,wherein the top of the cathode mass comprises parts which protrude abovethe sides of the cathode holder.
 60. The cathode unit of claim 48,wherein the top of the cathode mass does not extend above the sides ofthe cathode holder.
 61. The cathode unit of claim 48, wherein the top ofthe cathode mass comprises bodies such as tiles or blocks made of orcoated with an aluminium-wettable electrically-conductive material. 62.The cathode unit of claim 61, wherein said bodies protrude upwardly froma cathode mass made of an electrically-conductive material.
 63. Thecathode unit of claim 61, wherein the cathode mass is coated with analuminium-wettable material.
 64. The cathode unit of claim 48, whereinthe cathode mass is made mainly of carbonaceous material.
 65. Thecathode unit of claim 48, wherein the cathode mass comprises analuminium-wettable surface.
 66. The cathode unit of claim 48, whereinthe cathode is a drained cathode.
 67. The cathode unit of claim 48,wherein the cathode comprises bodies such as tiles or blocks.
 68. Anarrangement of interconnected aluminium production cells according toclaim 1, connected together by crossing-over busbars from one cell to anadjacent cell, wherein the busbar connected to the cathode holder of onecell is connected to the anode current supply of an adjacent cell. 69.The arrangement of claim 68, wherein pairs of cells are arrangedside-by-side in rows, the pairs of cells in each row being connected inparallel to corresponding pairs of cells in the adjacent rows.
 70. Thearrangement of claim 69, wherein each cell of side-by-side pair of cellsof one row comprises an anode current-supply superstructures, thesuperstructures of one row being connected together to a common anodebusbar, the cathode holders of two cells of each side-by-side pair ofcells of one row being connected together and to the common anode busbarof a corresponding side-by-side pair of cells of an adjacent row ofcells.
 71. The arrangement of claim 69, wherein the cells of eachside-by-side pair of cells of one row are placed close together withtheir common anode busbar situated therebetween, and the cells ofadjacent rows are spaced apart from one another leaving therebetween awalkway allowing access to all of the cells for servicing.
 72. Thearrangement of claim 68, wherein the anodes in each cell are arranged intwo rows of side-by-side anodes with pairs of side-by-side anodes in thetwo rows connected together, and wherein each crossing-over busbar isconnected to at least one pair of interconnected anodes.
 73. Thearrangement of claim 68, wherein each crossing-over busbar is connectedto two adjacent pairs of interconnected anodes.
 74. The arrangement ofclaim 68, wherein each crossing-over busbar is connected to at least twocrosswise current feeders in the cell bottom.
 75. A method ofmanufacturing the cathode unit of a cell for the production of aluminiumby the electroysis of an aluminium compound dissolved in a moltenelectrolyte, which cell comprises an outer mechanical structure formingan outer shell one or more cathodes and an electric and thermicinsulation separating the or each cathode from the outer shell, theouter shell and the electric and thermic insulation forming a recessthat houses the or each cathode, the or each cathode comprising an innerelectrically-conductive cathode holder supporting and substantiallycoextensive with a cathode mass, the cathode holder being connectedelectrically to a busbar, the or each cathode holder also serving todistribute current to its cathode mass, wherein the or each cathodeholder and the thereon supported cathode mass forming said individualcathode unit which is movable within said recess for insertion thereinand removal therefrom of said individual cathode unit, said methodcomprising providing a cathode holder, placing a cathode mass on thecathode holder so the cathode mass is substantially coextensive with,mechanically supported by and electrically connected to the cathodeholder, and connecting at least one current collector bar to theunderside of the cathode holder or to its side(s).
 76. A method ofinstalling a cathode unit in a cell for the production of aluminium bythe electrolysis of an aluminium compound dissolved in a moltenelectrolyte, which cell comprises a recess formed by an outer shell andan electric and thermic insulation for housing the cathode unit, saidmethod comprising placing an electrically-conductive cathode mass on acathode holder to form a cathode unit wherein current can be supplied tothe cathode mass by a current collector bar and distributed over thecathode mass by the cathode holder, installing the cathode unitcomprising the cathode holder and the cathode mass in said recess, andconnecting the cathode holder by a current collector bar to a busbaroutside the outer shell, the cathode holder and the theron supportedcathode mass being moved as an individual cathode unit within saidrecess during insertion and being removable therefrom.
 77. A method ofsupplying electric current to a cathode unit of a cell for theproduction of aluminium by the electrolysis of an aluminium compounddissolved in a molten electrolyte, which cell comprises a recess formedby an outer shell and an electric and thermic insulation for housing thecathode unit, the cathode unit comprising an innerelectrically-conductive cathode holder supporting and substantiallycoextensive with a cathode mass, the cathode holder and the theronsupported cathode mass being arranged to be movable as an individualcathode unit within said recess for insertion therein and removaltherefrom of said individual cathode unit, the method comprisingsupplying current via a cathode current collector bar to the bottom ofthe cathode mass, uniformly distributing the current supplied via thecurrent collector bar over the entire bottom of the cathode mass bymeans of the cathode holder, and passing the current from the cathodeholder into the cathode mass.
 78. A method of transforming an existingHall-Hëroult cell, comprising removing any used cathode after shuttingdown the cell, refurbishing and/or rebuilding an insulating liningformed by electric and thermic insulation, and installing one or morenew cathode units according to the method of claim
 76. 79. A method ofrenovating an aluminium production cell comprising a cathode unit afterthe cell has been taken out of service, which cell comprises a recessformed by an outer shell and an electric and thermic insulation forhousing the cathode unit the cathode unit comprising an innerelectrically-conductive cathode holder supporting and substantiallycoextensive with a cathode mass, the cathode holder and the theronsupported cathode mass being arranged to be movable as a individualcathode unit within said recess for insertion therein and removaltherefrom of said an individual unit, the method comprising removing thecathode unit from said recess and replacing it by inserting a new orrenovated cathode unit into said recess.
 80. A method of productingaluminium using a cell for the production of aluminium by theelectrolysis of an aluminium compound dissolved in a molten electroyte,which cell comprises an outer mechanical structure forming an outershell one or more cathodes and an electric and thermic insulationseparating the or each cathode from the outer shell, the outer shell andthe electric and thermic insulation a recess that houses the or eachcathode, the or each cathode comprising an inner electrically-conductivecathode holder supporting and substantially coextensive with a cathodemass, the cathode holder being connected electrically to busbar, the oreach cathode holder also serving to distribute current to its cathodemass, wherein the or each cathode holder and the thereon supportedcathode mass are movable as an individual cathode unit within saidrecess for insertion then and removal therefrom of said individualcathode unit, said method comprising supplying current to the cathodeunit via the current collector bar and the cathode holder whichdistributes the current uniformly to the cathode mass, the cathodeholdermaintaining the bottom of the cathode unit and the current collectorbars at practically the same electrical potential.
 81. The method ofproducing aluminium of claim 80, wherein the surface of the cathode massis maintained at a temperature corresponding to a paste state of theelectrolyte whereby the cathode mass is protected from chemical attack.82. The method of producing aluminium of claim 81, wherein the surfaceof the cathode mass is maintained at the selected temperature bysupplying gas via an air or gas space between the cathode holder and theelectric and thermic insulation.
 83. A method of starting up a cell forthe production of aluminium by the electrolysis of an aluminium compounddissolved in a molten electrolyte, which cell comprises an outermechanical structure forming an outer shells one or more cathodes and anelectric and thermic insulation separating the or each cathode from theouter shell, the outer shell and the electric and thermic insulationforming a recess that houses the or each cathode, the or each cathodecomprising an inner electrically-conductive cathode holder supportingand substantially coextensive with a cathode mass, the cathode holderbeing connected electrically to a busbar, the or each cathode holderalso serving to distribute current to its cathode mass, wherein the oreach cathode holder and the thereon supported cathode mass are movableas an individual cathode unit within said recess for insertion thereinand removal therefrom of said individual cathode unit, said methodcomprising heating the cathode unit by supplying beating and gas via anair or gas space provided between the cathode holder and the electricand thermic insulation.